This invention generally relates to electronic systems and in particular it relates to an all bipolar emitter couple pair transconductor for high speed operational amplifiers.
One of the most important parts in the design of operational amplifiers (opamps) is the input stage. This stage defines many characteristics that make an opamp unique when compared to others. The prior art emitter couple pair transconductor shown in FIG. 1 is the most common used input stage in opamps because it is well known and has excellent noise performance, low input offset voltage, high gain and very good common mode rejection ratio (CMRR). The circuit of FIG. 1 includes NPN bipolar transistors 20-23; PNP bipolar transistors 24 and 25; bias current Ibias; buffer 28; capacitor 30; tail current I_tail; inputs IN_POS and IN_NEG; and output OUT. Unfortunately, this input stage has poor slew rate (speed parameter) because the current available is limited by the biasing current. This is why other input stages and circuits are used in high speed applications instead of the emitter couple pair. One example is the current feedback opamp, which has a very good slew rate but poor gain, noise, CMRR and offset voltage.
The main limitation of the emitter couple pair transconductor in high speed opamps is the hyperbolic tangent (tanh) behavior of its output current. This is due to the fact that an emitter couple pair transconductor is biased in a class A mode, meaning that the output current available in signal transitions is limited to the tail current I_tail (biasing current) of the transconductor. The parameter that is directly affected by this is the slew rate, which is the measure of how fast an amplifier reacts to a very fast step signal. Slew rate in a transconductor is directly proportional to the current available (tail current I_tail) to charge the compensation capacitor 30 that is used for stability purposes. Since the current is limited, the slew rate will also be limited making this a first order limitation in a normal transconductor.
There is a way to increase the slew rate in a transconductor. Decreasing the value of the compensation capacitor does this. Unfortunately, changing this parameter alone will affect the stability of the transconductor by decreasing its phase margin. The correct way to decrease the compensation capacitor is accomplished by decreasing the transconductance of the transconductor while keeping its tail current and unity gain bandwidth fixed. This approach permits a lower value compensation capacitor while maintaining the bandwidth with good stability and higher slew rate. The well-known solution is to degenerate the emitters of the transistors in a normal transconductor. The price paid by doing this is a decrease in the open loop gain and an increase in noise therefore making the opamp less precise. In conclusion, the faster the transconductor is made the less precise it will be and vice versa.
Current feedback amplifiers are one solution to the slew rate limitation in traditional opamps. These amplifiers are very fast because they use as an input stage a second-generation current conveyor (CCII). Unlike the emitter couple pair transconductor, the CCII has unlimited output current behavior, that is, its output current has a hyperbolic sine (sinh) response to input voltages. This means that in signal transitions there will be an unlimited amount of current (in theory) to charge the compensation capacitor. In theory, slew rate in a CCII is infinite but in practice this will be limited by the internal capacitors of the transistors making this a second order effect instead of a first order effect like in the transconductor. The disadvantages of this topology are high input offset voltage, relatively low gain, low CMRR, in some topologies higher total harmonic distortion (THD), and the requirement of a feedback resistor to set the bandwidth and stability of the circuit.
An emitter coupled pair transconductor circuit includes: an emitter couple pair; and a tail current source coupled to the emitter couple pair wherein the tail current source provides a tail current that is a hyperbolic cosine function relative to a differential input signal. This solution transforms the output current of an emitter couple pair transconductor from a hyperbolic tangent to a hyperbolic sine by using a hyperbolic cosine comparator for the biasing. By doing this, the transconductor has a similar speed behavior to a second-generation current conveyor and is more precise. This transformation makes a regular transconductor very fast without changing its parameters.